Mailvelope

Mailvelope adds missing encryption and decryption features to the user interface of common webmail providers.
It supports the PGP encryption standard (OpenPGP, GPG) and is compatible with other PGP applications. Encrypt files on your hard drive with Mailvelope and send encrypted email attachments.
Mailvelope makes integrating security into your workflow seamless.
It comes preconfigured for common webmail providers like:
• Gmail™
• GMX™
• Outlook.com™
• Posteo™
• WEB.DE™
• Yahoo!™
But we also support other email providers or web applications which can be activated in Mailvelope with a one-click configuration step. For details, visit https://www.mailvelope.com/help
Mailvelope is open source and based on OpenPGP.js (https://openpgpjs.org), an OpenPGP library for JavaScript. The OpenPGP standard (based on PGP as originally developed by Phil Zimmermann) uses a combination of strong public key and symmetric encryption to secure electronic communication.
Follow us on Twitter: https://twitter.com/mailvelope
Get the source code on GitHub: https://github.com/mailvelope/mailvelope
Find the latest changelog at: https://github.com/mailvelope/mailvelope/blob/master/Changelog.md